This invention relates to fibre in the loop access networks, and in particular to fibre to the home (FTTH).
A characteristic of known FTTH networks is that customers tend to exist in groups situated geographically close to each other (say, within 200 meters), but the head end (or central office) may be some kilometers away for example, 5 km.
In networks where multiple nodes share a common medium, a multiple access protocol is needed to manage access to the medium such that individual nodes on the network can transmit their information successfully without interruption by transmission attempts from other nodes. Ethernet (IEEE standard 802.3) has adopted a ‘Carrier Sense’ with ‘Collision Detection’ protocol. When a node has data to transmit, it monitors the medium to check whether any other node is currently transmitting (Carrier Sense). If not, it is permitted to start transmission. Other nodes on the network might also start transmitting when they detect that the medium is free, but might not know of other nearly simultaneous transmissions by another node because of the inherent transmission delay of the medium. Simultaneous transmission results in a ‘collision’ which must be detected by all nodes simultaneously attempting to transmit. Once a collision is detected, each node ceases transmission and waits for a random time interval before sensing the medium and trying again.
Current implementations of Gigabit Optical Ethernet use point-to-point optical links to an ‘unbuffered repeater’ at the logical hub of the network. The repeater demodulates incoming signals from the point-to-point links and identifies a collision when optical activity occurs simultaneously on more than one input.
A disadvantage with this system is that it requires active electronics in the repeater which is not compatible with operator requirements to remove active electronics from street locations.
It is also known to use a passive optical star coupler to interconnect nodes in an optical network. A star coupler has a number of inputs and outputs where an optical signal on any input appears (attenuated) at all coupler outputs. Various techniques are known for constructing passive star couplers. One common technique is to interconnect a number of simpler fused fibre couplers, each having two inputs and two outputs, to form a larger matrix with the required functionality. For example, an eight by eight star coupler can be made up from 12 two by two couplers.
In a practical network, a coupler with an equal number of input and output ports, say eight input and eight output ports, would be used. Each node in the network connects to an input/output pair of ports. In this way, each node receives traffic sent by itself and all other nodes on the network. Carrier sense and collision detection can then be performed by each node to implement the multiple access protocol.
For collision detection to work properly it is desirable (in the case of Gigabit Ethernet, mandatory) that any collision should be detected before a transmitting node has finished the transmission. For high speed networks such as these using a passive optical star coupler, this results in a tradeoff between the physical size of the network and the minimum length of the transmitted packet. For a network running at a nominal bit rate of 1 Gbit/s and with an overall size of 5 km (typical of an access network) the minimum packet size to guarantee detection of a collision is around 6 kbytes.
A problem with this is that such a network is very inefficient for short packets, which must be extended to the minimum packet size to guarantee detection of any possible collisions. In practice, a significant proportion of packets (such as those used for voice and TCP/IP acknowledgements) will be short.
A further problem is that, the maximum permitted packet size on an Ethernet network is around 1500 bytes; any network including Ethernet segments cannot therefore use a larger packet size than this unless data is reformatted.
Collision detection also requires that any transmitter must be able to detect reliably when more than one node is transmitting simultaneously. In a network interconnected with a passive optical coupler, tolerances in the port to port coupler loss, differing attenuation in fibre connections between nodes and the passive coupler, and variations in node transmitter output powers combine to give a wide variation in optical signal level received at any node. It is then difficult to detect collisions, since a weak signal can be swamped by a strong one. Indeed, when practical tolerance levels are fully taken into account, it may not be possible to build a suitable receiver economically.
An earlier version of Ethernet, running at 10 Mbit/s, included an option to operate collision detection in the optical domain using a passive star coupler (known in the standard as ‘10baseFP’). In practice, this system is rarely used because of the difficulty of implementing a receiver capable of reliable collision detection. The problems associated with implementing such a receiver can be expected to be much more severe at a bit rate of 1 Gbit/s.